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A canon, in terms of a fictional universe, is a body of material
that is considered to be "genuine" or "official", that can be
directly referenced as, or as if it were, material produced by the
original author or creator of a series. New works set within that
universe are ostensibly constrained to be consistent with
pre-existing canon, though the issue is somewhat complicated by
several factors: pre-existing canon itself may also be subject to
retcon,
for instance, and some licensed works, such as movie or television
novelizations or
spin-off
novels, may not be considered "canonical" by some. Additionally,
adaptations of a work into other formats, such as feature film or television, may be
considered either non-canonical, or forming a separate canon; and
consistency with prior canon is not sufficient in and of itself to
make a work "canonical" - fan fiction, for instance, often follows
the original pre-existing canon but is not part of the canon.

Origins

The word "canon" originally referred to the books which the Church
officially chose to be included in the Bible (see also: canon law); by extension,
it can be taken as referring to the authoritative (albeit
metaphorical) "holy writ" of a fictional universe. However, the
practice of defining a "canon" in terms of a fictional world likely
derived from the concept of a literary canon, a specified collection of
works considered to be both representative and the best of a
particular form, genre or culture. In that more common
use of the word, works forming a canon do not have to bear any
strong relation to each other, apart from their perceived high
quality or historical influence.

The specific use of "canon" to describe the degree to which a
given work adheres to the standards of its fictional world appears
to have originated amongst devotees of the Sherlock Holmes
stories, as a way to distinguish between the original works of Arthur Conan
Doyle and adaptations of those works or original works by other
writers utilizing related characters and settings. However, much of
the interest in and controversy over issues of "canonicity" have
appeared in recent decades in the fan followings of films and television
shows, such as the science fictionfranchisesStar Wars and
Star Trek.

When the body of work nominally set in the same fictional
universe becomes large enough, it can happen that new material,
such as might be found in spin-off television shows, prequels
and books, contradicts earlier material. Such contradictions may be
a result of poor research, or an attempt to revise, or correct a
perceived error in, earlier material (see also: retcon). The question is
which material to favor and which to ignore when attempting to
resolve all the material into a consistent whole. Two simple
approaches are the "principle of first mention" in which
information in the original work provides a foundation which later
material must respect, and the revisionist model in which the
latest work always supersedes earlier material. However, the
situation can be much more complicated.

Nature of fictional
canons

The word canon can simultaneously refer to the considerations of
the publishers of a fictional series as well as what the fanbase
chooses to consider as authentic.

Generally, "Expanded Universes" are not
considered canonical; by analogy with the idea of a canon of Scripture (see Biblical canon),
such stories are considered "apocryphal". However, there are
exceptions. In the case of the Star Wars canon, the Expanded
Universe is canonical, though open to interpretation in a way which
the "gospel" of the films is not. Doctor Who, which began life as a
television series but has also been produced in prose, audio and
graphical formats, has never had a single author or authority to
pronounce on the issue of canon, and its fans run a spectrum
between those who consider only some parts of the television series
canonical and those who consider everything labeled as Doctor Who
canonical.

In addition, a story can belong to two overlapping canons. One
of the most obvious examples of this is Philip
José Farmer's Wold Newton family. Some (but not
all) of the adventures of Sherlock Holmes, Tarzan, Doc Savage, etc. are canonical in the Wold
Newton setting. This does not mean that the events of Farmer's
books are canonical from a Sherlockian perspective. Similarly, fans
of Laurie R.
King's novels of Sherlock Holmes and Mary Russell consider
all the Holmes stories to be canonical in King's setting.

The difference can be even less clear-cut than this. Current
Star Trek novels maintain a tight continuity with each
other, and avoid contradicting the television series. When a
Lost Era novel set between the movies and The Next
Generation features a younger version of a character
introduced in a Deep Space Nine novel, it is obvious there
is some sort of "canonical" novel-setting, even if the TV series is
not obliged to conform to it. This is where "fanon" and canon often
collide, especially when a TV series, movie or other officially
canonical source contradicts it. An example is the Trek
novel Starfleet: Year One, which appeared in print before
the TV series Star Trek: Enterprise was announced, but was
completely invalidated by the series. Generally, though, in the
case of televised fiction, only facts which appear in the
as-originally-aired version of a program are considered canonical
(including scenes cut from re-runs, but not including such things
as deleted scenes and scenes from unaired pilots and other such
material that "leaks out" over the Internet).

Furthermore, the issue is also complicated when the definition
of a canon changes well after the fictional universe is
established. As an example, a number of reference works for
Star Trek were published between 1970 and 1988 by Franz
Joseph and FASA Corporation. These books were
considered canonical at the time (some even made with the explicit
approval of Gene Roddenberry), sanctioned by Paramount
Pictures, and were used almost universally by novel and comic
book authors, as well as the production staff of the earlier
Star Trek movies (information from these manuals appeared
as background dialogue in some scenes, and many diagrams were used
as computer displays). However, in 1988, as part of the release of
Star Trek: The Next Generation, Gene Roddenberry and
Paramount Pictures changed their policies regarding canonicity and
stripped these books of their canonical status, as the new series
quickly made many changes and revelations which openly contradicted
earlier canonical books. Thus a book that was considered completely
canonical in 1985, such as The Star Trek Star Fleet Technical
Manual, would be considered non-canonical in 1995.

In recent years, complications have been created by the
increasing popularity of supplemental web content appearing on network's show sites. Deleted scenes and
webisode series, produced
by the studio are often considered canon,
while other materials, such as character profiles, stories, and
games, often produced by the networks themselves, generally are
not.

In some fictional universes, interviews and other communications
from authors are also considered canonical—like the
letters of J. R. R. Tolkien with relation to Middle-earth; also
items such as interviews, Internet chat sessions, and websites
(e.g., the website of J. K. Rowling in relation to the Harry Potter
series). This usually only happens in cases where all works in the
universe have the same author.

In almost all cases, fan fiction is not considered canonical, as
fan fiction is usually produced by amateurs. Sometimes, however,
events or characterizations portrayed in fan fiction can become so
influential that they are respected in fiction written by many
different authors, and may be mistaken for canonical facts by fans.
This is referred to as "fanon". An intentional inversion of the
exclusion of fan fiction came in Eric Flint's 1632 universe; in February 2000,
fans and other established authors were invited on the Internet
forum Baen's Bar to shape the multiverse, and the
fan-fic, once vetted, is itself published in the various Grantville Gazettes,
themselves under the direct editorial control of Flint and a
1632 editorial board. This is an ongoing process that
apparently will continue indefinitely, as the series continues to
enjoy burgeoning popularity.

Additionally, works of foreign origins (as is the case with most
Japanese-produced video games, manga or anime) may have certain details of the original
plot changed or modified during the adaptation from one language to
another. The person in charge of the adaptation may choose to write
an adaptation canon in addition to the original canon to maintain
consistency when adapting a possible later work such as a sequel or
a spinoff, although this is not always the case. An adapted version
of the same work can sometime deviate completely from its source
material, resulting a separate franchise from the original, as is
the case with the Macross and Robotech franchises.